Vat resin with additives for thiourethane polymer stereolithography printing

ABSTRACT

A method of three-dimensional stereolithography printing a thiourethane polymer part using the vat resin. Adding a resin to a vat of a three-dimensional stereolithography printer, the resin a liquid mixture including: a first type of monomer including two or more thiol functional groups, a second type of monomer including two or more isocyanate functional groups, a photolatent base, an anionic step-growth polymerization reaction inhibitor and a light absorber. The photolatent base is decomposable upon exposure to a light to form a non-nucleophillic base catalyst having a pKa greater than 7. The anionic step-growth polymerization reaction inhibitor has an acidic group configured to form an acid-base pair with the non-nucleophillic base. The light absorber has an absorbance in the liquid mixture that is greater than an absorbance of the photolatent base at a wavelength of the light used for the exposure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.16/170,342, filed by Gregory T. Ellson, et al. on Oct. 25, 2018,entitled “VAT RESIN WITH ADDITIVES FOR THIOURETHANE POLYMERSTEREOLITHOGRAPHY PRINTING,” which claims the benefit of U.S.Provisional Application Ser. No. 62/578,169, filed on Oct. 27, 2017,commonly assigned with this application and incorporated herein byreference.

TECHNICAL FIELD

This application is directed, in general, to a vat resin forthree-dimensional stereolithography printing of a thiourethane polymeras well as methods of preparing the vat resin and three-dimensionalstereolithography printing a thiourethane polymer part using the resin.

BACKGROUND

Stereolithography (SLA) is a well-established additive manufacturingprocess for forming three-dimensional (3D) parts. Photopolymer resinsused for SLA printing are often formulated for use with acrylatechemistry. The wide commercial and synthetic availability of acrylatemonomers allows for relatively cheap and rapid printing and the additionof resin additives such as radical and oxygen inhibitors to control thefree radical initiated chain-growth polymerization mechanism associatedwith polyacrylate formation.

However, SLA printed acrylic polymers can be brittle, thereby limitingtheir use for some structural applications. SLA printed acrylic polymerscan shrink during curing, resulting curing stresses when combined withthe early gelation at low conversion that is inherited for thepolyacrylate chain-growth polymerization mechanism. Curing stresses infully cured polyacrylates can cause undesired shape deformations in thefinal printed part. Secondary chemistries, such as epoxies,polyurethanes, and cyanate ester chemistries, have been attempted toimprove toughness and mechanical performance, but often at the cost ofrequiring extended thermal post-cure times before achieving mechanicalstability.

Therefore, there is a need to develop alternative polymer systems forSLA printing where the printed polymer can undergo rapid curing but hasreduced cure stresses and improved mechanical properties.

SUMMARY

One aspect of the disclosure is a vat resin for three-dimensionalstereolithography printing of a thiourethane polymer part comprising aliquid mixture. The liquid mixture includes a first type of monomer, asecond type of monomer, and a photolatent base. The first type ofmonomer includes two or more thiol functional groups, the second type ofmonomer includes two or more isocyanate functional groups. Thephotolatent base decomposes upon exposure to a light to form anon-nucleophillic base catalyst having a pKa greater than 7. The liquidmixture includes an anionic step-growth polymerization reactioninhibitor having an acidic group configured to form an acid-base pairwith the non-nucleophillic base. The liquid mixture include a lightabsorber having an absorbance in the liquid mixture that is greater thanan absorbance of the photolatent base at a wavelength of the light usedfor the exposure.

Another aspect of the disclosure is method of preparing a vat resin forthree-dimensional stereolithography printing a thiourethane polymerpart. The method comprises forming a liquid mixture. Forming the liquidmixture includes combining a first type of monomer and a second type ofmonomer, to form a monomer mixture, wherein the first type of monomerincludes two or more thiol functional groups and the second type ofmonomer includes two or more isocyanate functional groups. Forming theliquid mixture also includes adding an anionic step-growthpolymerization reaction inhibitor to the monomer mixture, the inhibitorhaving an acidic group configured to form an acid-base pair with thenon-nucleophillic base. Forming the liquid mixture also includes addinga photolatent base to the monomer mixture, the photolatent base beingdecomposable upon exposure to a light to form a non-nucleophillic basecatalyst having a pKa greater than 7. Forming the liquid mixture alsoincludes adding a light absorber having an absorbance in the liquidmixture that is greater than an absorbance of the photolatent base at awavelength of the light used for the exposure.

Another aspect of the disclosure is method of three-dimensionalstereolithography printing a thiourethane polymer part. The methodincludes adding a resin to a vat of a three-dimensionalstereolithography printer. The resin is a liquid mixture that includesthe first type of monomer, the second type of monomer, the photolatentbase and the light absorber.

BRIEF DESCRIPTION

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 presents a diagram of an example top-down three-dimensionalstereolithography printer configured to use the vat resin of the presentdisclosure;

FIG. 2 illustrates by flow diagram, selected aspects of an examplemethod of preparing a vat resin for three-dimensional stereolithographyprinting of a thiourethane polymer part in accordance with theprinciples of the present disclosure;

FIG. 3 illustrates by flow diagram, selected aspects of an examplemethod of three-dimensional stereolithography printing a thiourethanepolymer part in accordance with the principles of the presentdisclosure; and

FIG. 4 example tensile testing results of a thiourethane polymer partformed by an embodiment of the three-dimensional stereolithographyprinting method and using the vat resin in accordance with theprinciples of the present disclosure.

DETAILED DESCRIPTION

As part of the present disclosure, we developed a vat resin additivesappropriate for use in the 3D SLA printing of polythiourethane parts.Because polythiourethanes are cured using a non-nucleophilic Lewis base,free radical initiators and radical or oxygen inhibitors additivesdeveloped for use with acrylate based resins may not be appropriate foruse with polythiourethane 3D SLA printing methods and systems.

As further disclosed herein, embodiments of the vat resin can include acombination of resin additives including an anionic step-growthpolymerization reaction inhibitor (e.g., a cationic inhibitor) and alight absorber. These resin additives are provided in amounts to reduceor prevent polythiourethane photo-polymerization propagation in regionsof the vat resin that are outside of photo-defined development areas, tothereby improve the photo-definition of the 3D SLA printedpolythiourethane part.

FIG. 1 presents a diagram of an example top-down 3D SLA printer 100configured to use a vat resin 105 of the present disclosure. Asillustrated in FIG. 1 , embodiments of the printer 100 can include a vat110 for containing the resin 105 and an underlying liquid platform fluid115, a fluid pump 120, a build table 125 with a platform 130 alsolocated in the vat 110, and a light source 135. The liquid platformfluid 115 is denser than the resin and insoluble in the resin.Non-limiting examples of liquid platform fluids, sometimes referred toas a z-fluid, are presented in U.S. patent application Ser. No.14/261,320 (application 320'), and hereby incorporated by reference inits entirety.

The platform 130 and the resin 105 can be positioned relative to eachother in the vat 110 such that a thin layer (e.g., layer 105 a) of theresin 105 is located over the platform 130. A patterned light beam 140from the light source 135 (e.g., a laser or digital light projector) isthen directed into the vat 110 to selectively cure a targeted area 142of the thin layer 105 as part of forming a desired shape portion (e.g.,a patterned layer) of the polythiourethane part 145. The part 145 isformed as part of the photo-initiated catalysis of the anionicstep-growth polymerization reaction of thiol and isocyanatefunctionalized monomers in the targeted area of the resin 105 to formpolythiourethane.

We have found, however, that for such systems of 3D SLA printingpolythiourethane parts, the polymerization reaction can be undersiablypropagated into non-targeted regions of the resin 105 that are outsideof the target area 142 of the light beam 140, e.g., in regions of theresin 105 that are not within the thin layer 105 a.

For instance, photolatent base initiator molecules present in the resinand activated by the light beam (e.g., via photodecomposition of thephotolatent base molecule to form a non-nucleophillic base catalyst) candiffuse outside of the target area 142 and initiate the polymerizationof monomers present in non-targeted regions of the resin 105. Forinstance, light can scatter from the light beam 140 to non-targetedregions of the resin 105 to thereby active initiator molecules, which inturn can initiate the polymerization of monomers in these non-targetedregions.

As part of the present disclosure we have developed vat resinembodiments that include one or both of a combination of anionicstep-growth polymerization reaction inhibitor and light absorberadditives to mitigate the above-described undesired propagation of thepolymerization reaction.

One aspect of the disclosure is a vat resin (e.g., resin 105) forthree-dimensional SLA printing of a thiourethane polymer part (e.g.,part 145). Some embodiments of the resin can comprise a liquid mixturethat includes a first type of monomer, a second type of monomer, and aphotolatent base. The first type of monomer include two or more thiolfunctional groups, the second type of monomer includes two or moreisocyanate functional groups. The photolatent base decomposes uponexposure to a light (e.g., light beam 140) to form a non-nucleophillicbase catalyst having a pKa greater than 7. The resin further comprisesan anionic step-growth polymerization reaction inhibitor having anacidic group configured to form an acid-base pair with thenon-nucleophillic base. The resin also comprises a light absorber thathas an absorbance in the liquid mixture that is greater than anabsorbance of the photolatent base in the liquid mixture at a wavelengthof the light used for the exposure.

In some embodiments, the vat resin is substantially free of water (e.g.,less than 0.1 wt % or less than 0.01 wt % or less than 0.001 wt % insome embodiments). For instance anhydride or non-hydrated forms ofmonomers, photolatent base, inhibitor and light absorber are used in theliquid mixture of the resin.

While not limiting the scope of the disclosure by theoreticconsiderations, we believe that the storage lifetime of the resin can bedecreased by the presence of water, possibly due to the reaction betweenwater and isocyanate functional groups of the second type of monomer tothereby reduce the total number of isocyanate functionalized monomeravailable to participate in the step-growth polymerization reaction toform the polythiourethane part and may form a cyanuric acid byproductwhich may degrade the polythiourethane part's structure post-cure.Additionally, we believe that one of the reaction products between thewater and isocyanate functional groups may be carbamic acid, which inturn can form a cyanuric anhydride. We further believe that whilecyanuric anhydride may extend the polymer chain, when the chain breaksit will release CO₂ which in turn may degrade the printed polymer part'sstructure post-cure.

In some embodiments of the resin, a mole ratio of the photolatent baseto the anionic step-growth polymerization reaction inhibitor is in arange from about 5:1 to 15:1 and in some embodiments about 10:1. Suchratios are conducive to allowing the polymerization reaction to proceedin the target region (e.g., the target area 142 of layer 105 a) of lightillumination where the light beam (e.g., light beam 140) causesrelatively high concentrations of activated non-nucleophillic basecatalyst molecules (e.g., photodecomposed photolatent base molecules)and at the same time still provide enough inhibitor molecules in thenon-targeted regions to form acid-base pairs with activatednon-nucleophillic base catalyst molecules that have diffused out of thetarget region.

In some embodiments, the anionic step-growth polymerization reactioninhibitor is a strong organic acid and is non-oxidizing. That is, theinhibitor is substantially completely ionized (e.g., greater than 90%ionized and in some embodiments, greater than 99% ionized) in the liquidmixture of the resin and the inhibitor does not substantially oxidizethe thiol functional groups of the first type of monomer in the liquidmixture. Using an anionic step-growth polymerization reaction inhibitorthat is a strong acid facilitates the availability of acid groups thatcan form acid-base pairs with the activated non-nucleophillic basecatalyst molecules, e.g., diffused into the non-targeted regions of theresin. Using an anionic step-growth polymerization reaction inhibitorthat is non-oxidizing facilitates the storage life of the resin bymaintaining the availability of thiol functional groups that canparticipate in the polymerization reaction.

Non-limiting example embodiments of the anionic step-growthpolymerization reaction inhibitor include: octanoic acid,methanesulfonic acid, trifluoromethanesulfonic acid or carboxlic acid.For example in some embodiments, the anionic step-growth polymerizationreaction inhibitor of p-toluenesulfonic acid has a concentration in theliquid mixture in a range from about 0.001 to 0.2 wt %, and in someembodiments, in a range from about 0.05 to 0.2 wt %.

In some embodiments, the light absorber in the liquid mixture has anabsorbance that is at least about 1 percent higher, and in someembodiments, 5 percent higher and in some embodiment 10 percent higherand in some embodiments, 20 percent higher, than the absorbance of thephotolatent base at the wavelength of the light that the resin isexposed to.

Such embodiments are conducive to the photolatent base moleculesabsorbing enough of the light and thereby be activated non-nucleophillicbase catalyst molecules in the target area to catalyze thepolymerization reaction and at the same time still permit the lightabsorber to absorb light scattered into the non-target areas of theresin and thereby reduce the amount light available to activate thephotolatent base molecules in the non-target areas.

In some embodiments the light absorber has a high molar extinctioncoefficient at the wavelength used to activate the photolatent base(e.g., at least about 10000 M⁻¹ cm⁻¹). Having a high molar extinctioncoefficient is conducive to using low (e.g., millimolar or lowerconcentrations) of the light absorber in the fluid mixture of the resin,which in turn is conducive to having the light absorber fully dissolvein the mixture, e.g., to mitigate light scattering effects frompartially precipitated light absorbers.

Consider an example where the light absorber is or includes2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole (molar extinctioncoefficient equal to about 47000 M⁻¹ cm⁻¹ at about 373 nm) with aconcentration in the fluid mixture that is in a range from about 0.001to 1 wt %. At a concentration of about 1 wt % (e.g., about 23 mM) theabsorbance in the fluid mixture would equal about 1080. At a fluidconcentration of about 0.01 wt % (e.g., about 0.23 mM) the absorbance inthe fluid mixture would equal about 10.8 and at a fluid concentration ofabout 0.001 wt % (e.g., about 0.023 mM) the absorbance in the fluidmixture would equal about 1.08

Based on the present disclosure one skilled in the pertinent art wouldappreciate that the light absorber could be molecules selected have asufficiently high molar extinction coefficient in the UV or in thevisible light range to be soluble in the fluid mixture and have anabsorbance that is greater than the absorbance of the photolatent baseand the wavelength of light that is used to activate the photolatentbase.

In some embodiments of the resin, the photolatent base is or includes5-(2′-(methyl)thioxanthone)-1,5- diazabicyclo[4.3.0]non-5-enetetraphenylborate. Other non-limiting examples of other photolatentbases, are presented in U.S. patent application Ser. No. 15/458,220(application 220'), and hereby incorporated by reference in itsentirety.

In some embodiments, the first type of monomer in the resin is orincludes one of more of: 2,2′-(ethylenedioxy)diethanethiol,decanedithiol, hexanedithiol, glycol dimercaptoacetate, glycoldimercaptopropionate, thiobisbenzenethiol, xylene dithiol,pentaerythritol tetramercaptoacetate, pentaerythritoltetramercaptopropionate, dipentaerythritol hexamercaptopropionate,trimethylolpropane trimercaptoacetate, trimethylolpropanetrimercaptoacetate, or tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate.

In some embodiments, the second type of monomer in the resin is orincludes one of more of: hexamethylene diisocyanate,trimethylhexamethylene diisocyanate, diisocyanatooctane, isophoronediisocyanate, xylene diisocyanate, toluene diisocyanate, phenylenediisocyanate, bis(isocyanatomethyl)cyclohexane, 4,4′-methylenebis(phenylisocyanate), 4,4′-methylenebis(cyclohexyl isocyanate), ortris(6-isocyanatohexyl)isocyanurate.

Other non-limiting examples of other first and second type monomers arepresented in the application 220'.

Based upon the present disclosure one skilled in the pertinent artswould appreciate that the amounts of inhibitor and light absorberpresent in the resin would depend upon the on the amount of thephotolatent base initiator present in the resin as well as theabsorbance of the photolatent base at the wavelength of light beam usedto activate the photolatent base exposure to form the non-nucleophillicbase catalyst.

Another aspect of the disclosure is a method of preparing a vat resinfor 3D SLA printing a thiourethane polymer part. FIG. 2 illustrates byflow diagram, selected aspects of an example method 200 of preparing thevat resin in accordance with the principles of the present disclosure.

With continuing reference to FIG. 2 throughout, some embodiments of themethod 200 can comprise forming a liquid mixture (step 205). Forming theliquid mixture (step 205) includes combining a first type of monomer anda second type of monomer to form a monomer mixture (step 210). Anycombination of the first and second types of monomers disclosed hereincould be mixed together to form a homogenous monomer mixture. Forexample, the first type of monomer can include two or more thiolfunctional groups and the second type of monomer can include two or moreisocyanate functional groups.

Forming the liquid mixture (step 205) also includes adding an anionicstep-growth polymerization reaction inhibitor to the monomer mixture(step 215), e.g., any anionic step-growth polymerization reactioninhibitor that has an acidic group configured to form an acid-base pairwith the non-nucleophillic base.

Forming the liquid mixture (step 205) also includes adding a photolatentbase to the monomer mixture (step 220), e.g., any disclosed photolatentbase that decomposes upon exposure to a light to form anon-nucleophillic base catalyst having a pKa greater than 7.

Forming the liquid mixture (step 205) also includes adding a lightabsorber to the monomer mixture (step 225), e.g., any light absorberthat in the liquid mixture that will have an absorbance that is greaterthan an absorbance of the photolatent base at a wavelength of the lightused for the exposure.

Embodiments of the method 200 can include any combination of sequentialadditions of the anionic step-growth polymerization reaction inhibitor,the photolatent base and the light absorber to the monomer mixture, or,adding any two or all three of these to the monomer mixturesimultaneously to form a homogenous liquid mixture.

Still another aspect of the disclosure is a method of three-dimensionalstereolithography printing a thiourethane polymer part. FIG. 3illustrates by flow diagram, selected aspects of an example method 300of three-dimensional stereolithography printing a thiourethane polymerpart in accordance with the principles of the present disclosure

With continuing reference to FIGS. 1 and 3 throughout, some embodimentsof the method 300 can comprise adding a resin 105 to a vat 110 of athree-dimensional stereolithography printer 100 (step 305). The resin105 can comprise any of the embodiments of the liquid mixture includingthe first type of monomer, the second type of monomer, the photolatentbase, the anionic step-growth polymerization reaction inhibitor and thelight absorber disclosed herein.

Embodiments of the method 300 can also comprise positioning (step 310)the resin 105 or a platform 130 of a build table 125 located in the vat110 such that a thin layer 105 a of the resin 105 is located over theplatform 130.

In some embodiments, as part of positioning (step 310), the build table125 can be moved to flow a thin layer of the resin (e.g., layer 105 a ofthickness 100 to 500 microns) on top of a previously cured layer of thepart 145 (e.g., layer 145 a). In other embodiments, as part ofpositioning (step 310), the amount of liquid platform 115 in the vat 110can be increased by adding liquid platform 115 to the vat 110 via thepump 120 (e.g., a syringe or peristaltic pump) raise the level of resinin the vat and thereby to flow a thin layer of the resin 105 a on top ofthe previously cured layer 145 a. In some embodiments a portion of theliquid platform 115 can be subtracted from the vat 110 via the pump 120to lower the level of the resin 105 in the vat 110 but leave the thinlayer of resin 105 a on top of the previously cured layer 145 a.

In yet other embodiments, as part of positioning (step 310), the lightsource 135 can be moved and/or the focal plane of the projected lightbean 140 can be adjusted such that only the thin layer 105 a of theresin 105 is exposed to the light beam 140 (step 312).

Embodiments of the method 300 can also comprise exposing (step 315) atarget area 142 of the thin layer 105 a to a beam of the light 140, thelight having a pattern corresponding to one cross-section of thethiourethane polymer part such that the photolatent base in the lightexposed target area of the resin decomposes to form thenon-nucleophillic base catalyst and thereby catalyzes polymerization ofthe first and second types of monomer together to form a patterned layerthe thiourethane polymer part 145.

In embodiments of the method 300, the inhibitor and light absorberadditives in the resin 105 can substantially prevent the polymerizationof the first and second types of monomer in areas of the resin lyingoutside of the target area 142.

Based on the present disclosure one skilled in the pertinent arts wouldunderstand how the process of selectively exposing target areas 142 ofsuccessive thin layers (e.g., layer 105 a) of resin 105 to the lightbeam 140 can be repeated until the final three-dimensional part 145 isformed. The part 145 is then removed from the vat 110, cleaned, andpost-cured.

Experiments

To illustrate various features of the disclosure, the 3D SLA printing ofan example polythiourethane part was tested using an embodiment of thevat resin and an embodiment of the method of printing in accordance withdisclosure.

3D SLA printing was performed using a B9 Creator, Formlabs Form 2,Somerville, Mass.) and Carbon M1 digital light projector adjusted toemit a light bean at 385 nm.

Type one and type two monomers 2,2′-(ethylenedioxy)diethanethiol, andhexamethylene diisocyanate, and tris(6-isocyanatohexyl)isocyanurate,respectively, were mixed with a photolatent base of5-(2′-(methyl)thioxanthone)-1,5-diazabicyclo[4.3.0]non-5-enetetraphenylborate, a reaction anionic step-growth polymerizationreaction inhibitor of p-toluenesulfonic acid hydrate, and a lightabsorber (e.g., UV light absorber) of2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole) until a homogenousliquid mixture was obtained.

In various embodiments of the resin, the amount of the isocyanatefunctional groups that was contributed bytris(6-isocyanatohexyl)isocyanurate was in a range from about 5% andabout 100% of the isocyanate functional groups, with the remainder ofisocyanate functional groups contributed from hexamethylenediisocyanate. The amount of photolatent base used was in a range fromabout 0.1 wt % to about 1 wt % of the total monomer mass. The amount ofinhibitor used was in a range from about 0% wt % and about 0.1 wt %. Theamount of light absorber used was in a range from about 0.01 wt % toabout 0.5 wt % of UV absorber used.

Following printing, the part was removed from the build plate and washedwith hexanes before undergoing UV post-curing for one hour at 85° C.

Tensile testing of embodiments of such print parts was performed todemonstrate that the printed parts have a have high toughness in tension(see e.g., FIG. 4 ).

Those skilled in the art to which this application relates willappreciate that other and further additions, deletions, substitutionsand modifications may be made to the described embodiments.

What is claimed is:
 1. A method of three-dimensional stereolithographyprinting a thiourethane polymer part, comprising: adding a resin to avat of a three-dimensional stereolithography printer, wherein the resinis a liquid mixture including: a first type of monomer including two ormore thiol functional groups, a second type of monomer including two ormore isocyanate functional groups, a photolatent base, wherein thephotolatent base is decomposable upon exposure to a light to form anon-nucleophillic base catalyst having a pKa greater than 7; an anionicstep-growth polymerization reaction inhibitor, the inhibitor having anacidic group configured to form an acid-base pair with thenon-nucleophillic base; and a light absorber having an absorbance in theliquid mixture that is greater than an absorbance of the photolatentbase at a wavelength of the light used for the exposure.
 2. The methodof claim 1, further including positioning the resin or a platform of abuild table of the printer located in the vat such that a thin layer ofthe resin is located over the platform.
 3. The method of claim 1,further including exposing a target area of the thin layer to a beam ofthe light, the light having a pattern corresponding to one cross-sectionof the thiourethane polymer part such that the photolatent base in thelight exposed target area of the resin decomposes to form thenon-nucleophillic base catalyst and thereby catalyzes polymerization ofthe first and second types of monomer together to form a patterned layerthe thiourethane polymer part.
 4. The method of claim 1, wherein thepolymerization of the first and second types of monomer in areas of theresin lying outside of the target area is substantially prevented. 5.The method of claim 1, wherein the vat resin is substantially free ofwater.
 6. The method of claim 1, wherein a mole ratio of the photolatentbase to the anionic step-growth polymerization reaction inhibitor is ina range from about 5:1 to 15:1.
 7. The method of claim 1, wherein thelight absorber in the liquid mixture has an absorbance that is at leastabout 1 percent higher than the absorbance of the photolatent base atthe wavelength of the light that the vat resin is exposed to.
 8. Themethod of claim 1, wherein the light absorber in the liquid mixture hasa molar extinction coefficient of at least about 10000 M⁻¹ cm⁻¹ at thewavelength used to activate the photolatent base.